The automotive industry has actively sought improved emissions reduction, including reduction in emissions due to gasoline evaporation. Gasoline includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility C8 to C10 hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister called an “evap canister” that contains an adsorbent material such as activated carbon granules.
As the fuel vapor enters an inlet of the canister, the fuel vapor diffuses into the carbon granules and is temporarily adsorbed. The size of the canister and the volume of the adsorbent material are selected to accommodate the expected fuel vapor generation. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. An engine control system may use an engine control module (ECM), a powertrain control module (PCM), or other such controller to optimize fuel efficiency and minimize regulated emissions. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture to consume the desorbed fuel vapor. To optimize fuel efficiency it is desirable to take this secondary air/fuel source into account. Presently, however, canister purge fuel and air are not metered, and so the ECM has no data to use in adjusting the fuel and air to the engine. Exhaust oxygen sensor feedback control is used to adjust fuel control during canister purge. Feedback control, as it is after the fact, is not very effective in exhaust emissions control. Stringent exhaust emission regulations, however, require ever more careful control of the air/fuel ratio in the engine. On the other hand, more stringent evap emissions regulations require increased purge air rates, meaning even more un-metered air entering the engine.
Additionally, the amount of adsorbed fuel vapor in the canister varies during the desorption process. The rate at which fuel vapor is drawn from the canister will decrease as more and more is removed until finally all of the fuel will have been desorbed from the canister. It would be desirable to enable the engine or powertrain control module (“controller”) to take into account the amount of fuel vapor drawn from the storage container in optimizing fuel efficiency and minimizing emissions and to be able to adjust for the decrease in fuel vapor from the storage canister as the adsorbed fuel is depleted.
One way to provide to the controller the information of fuel vapor and purge air drawn from the storage container would be to measure directly the amount of hydrocarbon and air being drawn from the storage canister using a purge hydrocarbon sensor so that the engine controller can reduce the fuel from the fuel tank injected into the engine and air intake of the engine accordingly. This approach will result in feed forward control that is very effective in exhaust emission control, but would require adding an expensive purge sensor to the engine.
It would thus be useful to have a method of predicting the amount of hydrocarbon in the air drawn through the canister into the engine for better feed forward fuel control without adding expensive equipment to the engine.